Electric motors

ABSTRACT

An electric motor includes one or more separate coil sets arranged to produce a magnetic field of the motor. The electric motor also includes a plurality of control devices coupled to respective sub-sets of coils for current control. A similar arrangement is proposed for a generator. A coil mounting system for an electric motor or generator includes one or more coil teeth for windably receiving a coil for the motor and a back portion for attachably receiving a plurality of the coil teeth. A traction control system and method for a vehicle having a plurality of wheels independently powered by a respective motor. A suspension control system and method for a vehicle having a plurality of wheels, each wheel being mounted on a suspension arm of the vehicle and being independently powered by a respective motor.

RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 12/373,202, which is a US 371 national stage entry ofInternational Application No. PCT/GB2007/002651, filed Jul. 13, 2007,which claims priority to United Kingdom Patent Application No. GB0613941.4, filed Jul. 13, 2006, the teachings each of which areincorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to electric motors or generators and to a tractioncontrol system.

BACKGROUND OF THE INVENTION

Known electric motor systems typically include a motor and a controlunit for controlling power to the motor. Known types of electric motorinclude the induction motor, brushless permanent magnet motor, switchedreluctance motor and synchronous slip ring machine. Three phase electricmotors are the most common kind of electric motor available.

FIG. 1 shows a schematic representation of a typical three phase motor.In this example, the motor includes three coil sets. Each coil setproduces a magnetic field associated with one of the three phases of themotor. In a more general example, N coil sets can be used to produce anN-phase electric motor. Each coil set can include one or more sub-setsof coils which are positioned around a periphery of the motor. In thepresent example, each coil set includes four such sub-sets—the coilsub-sets of each coil set are labelled 14, 16 and 18, respectively inFIG. 1. As shown in FIG. 1, the coil sub-sets 14, 16, 18 are evenlydistributed around the motor 10 to co-operate in producing a rotatingmagnetic field within which a central rotor 12, which typicallyincorporates one or more permanent magnets, can rotate as shown by thearrow labelled C. The coil sub-sets of each coil set are connectedtogether in series as shown by the connections 24, 26 and 28 in FIG. 1.This allows the currents in the coils of each coil set to be balancedfor producing a substantially common phase. The wires of each coil setare terminated as shown at 34, 36 and 38 in FIG. 1. Typically, one endof the wire for each coil set is connected to a common referenceterminal, while the other wire is connected to a switching system forcontrolling the current within all of the coils of that coil set.Typically then, current control for each coil set involves controlling acommon current passing through a large number of coils.

As shown in FIG. 2, each coil sub-set can include one or more coils. Inparticular, FIG. 2 shows the coils 24A, 24B in one of the coil sub-sets14. In this example, there are two coils per coil sub-set. The two coilsare wound in the opposite directions, and are interconnected so that thecurrent flowing in each coil is substantially the same. As the poles ofthe rotor 12 sweep across the coils 24A, 24B, switching of the currentin the coils 24A, 24B can produce the appropriate magnetic field forattracting and repelling the rotor for continued rotation thereof. Themagnetic field produced by the two oppositely wound coils 24A, 24B isreferred to as belonging to the same phase of this three phase motor.Every third coil sub-set arranged around the periphery of the motor 10produces a magnetic field having a common phase. The coils and theinterconnections may typically comprise a single piece of wire (e.g.copper wire) running around the periphery of the motor and wound intocoils at the appropriate locations.

For a three phase electric motor, the switching system is almostinvariably a three phase bridge circuit including a number of switches.

Typical power electronic switches including the Metal Oxide SiliconField Effect Transistor (MOSFET) and the Insulated Gate BipolarTransistor (IGBT) exhibit two principal losses: switching losses andconduction losses.

While switching losses decrease with switching speed, a faster switchingspeed also leads to increased electromagnetic interference (EMI) noise.This problematic trade off between switching speed and EMI noise iscompounded at higher power ratings (e.g. for a larger motor), sincelarger switches are required. The inductance associated with a powerswitch and its connection system increases with the physical size of theswitch. This inductance impacts the switching speed of the power deviceand the switching speed of a power device is typically therefore limitedby its physical size. Accordingly, for high power ratings largerswitches must be used, but larger switches involve slower switchingspeeds and therefore larger switching losses. Moreover, the cost of apower device increases roughly with the square of the size of thedevice. Conduction losses also increase with increased power.

Including switching losses and conduction losses, the total losses areapproximately proportional to the square of the power. This imposesserious thermal management problems for the motor since, for example, adoubling of the power leads to a four fold increase in thermal losses.Extracting this heat without elevating the temperature of the deviceabove its safe operating level becomes the limiting factor in what powerthe device can handle. Indeed, today larger power devices havingintrinsic current handling capabilities of, for example, 500 A arerestricted to 200 A due to thermal constraints.

Consider a conventional three phase motor with a given power rating. Ifa larger power rating is desired, this can be achieved by producing amotor with a larger diameter. For a larger motor diameter, theperipheral speed of the rotor increases for a given angular velocity.For a given supply voltage this requires that the motor coils to have areduced number of turns. This is because the induced voltage is afunction of the peripheral speed of the rotor and the number of turns inthe coils. The induced voltage must always be at or below the supplyvoltage.

However, the reduced number of turns in the coils leads to a reducedinductance for the motor, since the inductance of the motor isproportional to the square of the number of turns.

Almost all electronic control units for electric motors today operate bysome form of pulse width modulation (PWM) voltage control. PWM controlworks by using the motor inductance to average out an applied pulsevoltage to drive the required current into the motor coils. Using PWMcontrol an applied voltage is switched across the motor windings for aminimum period dictated by the power device switching characteristic.During this on period, the current rises in the motor winding at a ratedictated by its inductance and the applied voltage. The PWM control isthen required to switch off before the current has changed too much sothat precise control of the current is achieved.

As discussed above, the use of larger power devices leads to a slowerswitching speed, while a larger motor also has a lower inductance. Forhigher power motors, these two factors inhibit the effectiveness of PWMas a control system because the current in the motor coils rises morerapidly (due to the low inductance of the motor due to the fewer numberof turns in the coils) but the PWM control is more coarse (due to theslow switching speed achievable using high power switching devices).

A known solution to this problem is to introduce additional inductancein the motor in the form of current limiting chokes in series with themotor windings. This added inductance increases the rise time of thecurrent in the motor coils. However, the chokes are typically as largeor larger than the motor itself and as they carry the full current theydissipate a large additional heat loss as well as being a substantialextra volume, weight and cost.

Other problems with known motors relate to their manufacture. Asdescribed above in relation to FIG. 1, motor construction typicallyinvolves using a single length of wire to produce the windings for eachphase of the motor. The wire runs around a periphery of the motor andcoils are wound at the appropriate locations for producing a phase ofthe magnetic field of the motor. Winding the coils of the motor, as wellas terminating the connections between each coil sub-set interspersedaround the motor periphery is a labour intensive task. The thick wire(e.g. copper wire) typically used in motor windings is difficult tomanipulate and in many motor designs, access to the innards of the motorfor installing the coils and their interconnections is limited. Knowncoil mounting systems are also bulky and have limited heat dispersingcapabilities.

Vehicle traction control can be used for minimizing the risk of skidswhich can occur while the vehicle is moving. A vehicle relying on wheeltraction to provide a resultant locomotive force suffers from thephenomenon of wheel skid. Steering skids can also occur. In a steeringskid, the motion of the vehicle is out of alignment with that of thefront wheels (commonly known as under-steer) or the rear wheels(over-steer).

In general, the onset of a skid is not a sudden event, but starts with adegree of wheel slip, which then builds up to a full wheel skid. Theamount of force needed to produce a wheel slip or skid can be calculatedby the weight on the wheel multiplied by a coefficient of frictionbetween the tyre and the road surface. If this force is exceeded, then awheel slip or skid will occur. At forces just below the force at whichwheel slip or skid can occur, maximum drive performance is beingobtained while the wheel is still in grip. Traction control systemsgenerally aim to allow operation in this region, whereby maximum forcecan be applied to the wheels with allowing wheel slip or skid to occur.

In known systems, torque is applied to the wheels of a vehicle from acentral internal combustion engine through a drive shaft anddifferential gears. Traction control is normally applied throughmodulating the brake discs pressure (for braking) or by modulating aslip clutch mechanism by each wheel (for acceleration). These tractioncontrol systems require expensive mechanical parts and do not alwaysprovide the best performance. For example, ABS brakes tend to shudderviolently as they are operated on a crude on/off basis. Slip clutcheshave an effect on left/right torque balancing from the engine.

SUMMARY OF THE INVENTION

Aspects of the invention are defined in the accompanying claims. Forease of understanding, aspects of the invention are given subheadingsbelow corresponding to accompanying parts of the description but, forthe avoidance of doubt, these aspects may be used together in a singleembodiment of the invention.

Coil Control

According to an aspect of the invention, there is provided an electricmotor. The motor includes one or more separate coil sets arranged toproduce a magnetic field of the motor. Each coil set includes aplurality of coil sub-sets. Each coil sub-set includes one or morecoils. The magnetic field produced by the coils in each coil set have asubstantially common phase. The motor also includes a plurality ofcontrol devices each coupled to a respective coil sub-set forcontrolling a current in the coils of that respective coil sub-set. Eachcontrol device is operable without requiring an input synchronisationsignal.

Control of the currents in the coils of the motor is enhanced becausethe current in each coil sub-set can be controlled independently of thecurrent in another coil sub-set. Because all of the coils of each coilset are not connected in series, the coil or coils of each coil sub-setcan have a larger number of turns.

The increased number of turns in each coil increases the overallinductance of the motor. This means that lower currents can be used inthe coils of each coil sub-set, which leads to fewer heat dissipationproblems, and which allows smaller switching devices to be used. The useof smaller switching devices in turn allows for faster switching speedsand lower switching losses.

The control devices can include one or more switches for applying apulsed voltage to the one or more coils of a coil sub-set. PWM controlof the currents in the motor coils can be enhanced due to the increasednumber of turns which can be included in the coils. Because smallerswitching device can be used, significant savings in cost, weight andheat dissipation can be made.

Some of the control devices can include means for monitoring a back EMFwithin the coils of that coil sub-set. The control device can adjust apulse of the pulsed voltage (e.g. a width of the pulse) in response tothe monitored back EMF for high speed power control. The control devicescan operate independently of one another because each control devicecomprises sufficient logic to determine the position of the rotor and soto apply the appropriate voltage to control the current in therespective coil subset. The control devices can receive a demand signalfrom an external device, such as a brake pedal sensor, and applyappropriate coil control based on the coil characteristics, the positionof the rotor and the demand signal.

Since smaller components (e.g. switching devices) can be used, they canbe housed within a casing of the motor, in contrast to known systemsusing large, bulky switching devices. For example, the control devicescan be located adjacent their respective coil sub-sets within the motorthereby simplifying termination of the coil windings. The casing of themotor can include one or more apertures dimensioned such that thecontrol devices can be accessed one at a time, depending on theorientation of the rotor/casing and the control devices.

A common control device can be provided to coordinate the operation ofthe plurality of control devices. For example, the common control devicecan be used to coordinate the switches within the plurality of controldevices to ensure that the switching of the currents in each coil set issubstantially in phase. In this way, the control devices can operate toemulate a motor in which the coils of each coil set are all connected inseries. Alternatively, each control device can control its phaserelationship by detecting a position of the rotor of the motor and inthis way provide for complete parallel operation, without dependence oncentral controller. This would enhance immunity to any single failurewithin the motor.

The common control device can be operable to selectively disable one ormore of the control devices to allow fractional power operation.

According to another aspect of the invention, there can be provided amethod of operating an electric motor of the kind described above. Themethod includes using the plurality of control devices to supply powerto the coils of the respective coil sub-sets for producing the magneticfield of the motor.

It will be appreciated that an electric generator is structurallysimilar to an electric motor, and that some of the considerationsdiscussed above can also be employed in a novel generator.

According to a further aspect of the invention, there can be provided anelectric generator. The generator includes one or more separate coilsets arranged to produce an induced current due to a magnetic fieldproduced within the generator. Each coil set includes a plurality ofcoil sub-sets. Each coil subset includes one or more coils. The currentproduced in the coils of each coil set have a common phase.

The generator also includes a plurality of power outputs each coupled toa respective coil sub-set for outputting current produced in the coilsof said respective coil sub-set.

Position Sensing

A further aspect of the invention is the use of an iron focussing ringto assist in the alignment of magnetic fields used to detect theposition of a rotor with respect to a stator.

Braking Arrangement

According to a further aspect of the invention, there is provided anelectric motor configurable to operate in a braking mode. The motorincludes one or more coil sets arranged to produce a magnetic field.Each coil set including a plurality of sub-sets. Each coil sub-setincluding one or more coils. The motor also includes a plurality ofcontrol devices each coupled to a respective coil subset for controllinga current in the one or more coils of the respective coil sub-set.

The control devices being operable by current drawn from the coils whenin a braking mode.

Since the control devices can operate from current drawn from the coils,a fail-safe braking arrangement is provided as the control devices cancontinue to operate (and thereby control braking) even in the event offailure of the power supply. Preferably, each control device is arrangedso that it is operable from current from one respective sub-set of coilswhen in a braking mode. This ensures that there is redundancy built intothe braking arrangement, as, in the event of failure of a coil, othercoils and control devices would still be operable to provide a brakingforce.

The motor preferably also includes a capacitance coupled between thecoils and a connection for a power supply. The capacitance ensures thatcurrent can continue to be supplied to the control devices when atransition occurs between a power consuming mode and non-power consumingmode. The motor also includes a resistance selectively coupled to thecontrol devices such that in an emergency braking mode power from thecoils may be consumed by the resistance. An emergency braking mode isone in which a power supply is unable to receive power from the coils,for example, because the power supply such as a battery has failed, abattery is full or a connection has failed. The resistance is preferablyarranged very close to the control devices and coils thereby reducingthe risk of connection failure.

An aspect of the invention also provides a control arrangement for usewith the motor described above, comprising a mechanical brake controldevice being connected to a plurality of brake controller circuits, eachbrake controller circuit being coupled to a respective electric motor.In the example of a vehicle, the mechanical brake control device is abrake pedal and separate brake controller circuits are connected to thebrake pedal so as to provide redundancy such that in the failure of anyone brake controller circuit, other circuits are operable to control thebraking force provided by one of the motors connected to drive arespective wheel of the vehicle.

Coil Switching

The plurality of control devices can be configured to provide staggeredswitching of the currents in the coils of the motor within a polyphasecycle of the motor. This allows EMI noise to be mitigated by spreadingthe switching events across a given time period. This aspect operates bycausing PWM switching events to occur at different times in differentcoils.

Embodiments of this invention can be employed to produce, for example, alinear or rotary motor. For example, the coil sets can be arranged toproduce a rotating magnetic field, and the motor can include a magnetmounted to rotate within the rotating field.

Coil Mounting

According to another aspect of the invention, there can be provided acoil mounting system for an electric motor. The system includes one ormore coil teeth for windably receiving a coil for the motor. The systemalso includes a back portion for attachably receiving a plurality of thecoil teeth.

The coil teeth provide a means by which a coil winding can be preparedseparately and away from the motor and other coils. In this way, eachcoil can be prepared relatively easily, with easy access to the coiltooth and without the other components of the motor getting in the way.The back portion provides a means by which the coils of mounting system,once wound, can be attached in a desired arrangement for producing theappropriate magnetic field.

The coil tooth can include an elongate arm for windably receiving thecoil. Again, this simplifies the task of producing the coils.

The coil tooth can include a shaped attachment portion, and the backportion can include a correspondingly shaped receiving portion. This canprovide a simple and robust attachment between the teeth and backportion. The attachment portion of the coil tooth can have two fingersin a V shaped configuration. The fingers can be angled to runsubstantially along magnetic field lines produced by a coil wound on thetooth, thereby to reduce interference of the fingers with the field.

A plurality of interconnected back portions can be provided. Again thisprovides further flexibility in designing and constructing the motor.For example, multiple back portions, each carrying a number of teeth,can be assembled separately and then connected together to form a largerback portion and tooth arrangement for the motor. The back portions canbe stacked one above the other, and the back portions in each layer canbe interconnected in such a way that the interconnections are staggeredsuch that they do not coincide vertically.

This increases construction strength. The interconnections between backportions can be provided via the teeth.

The back portion can be shaped and dimensioned to produce a desiredarrangement for the coil teeth attached thereto. For example, the backportion can be arcuate. This would allow a circular motor to beconstructed incorporating the mounting system. The back portion and/orthe coil tooth have a laminated construction.

According to a further aspect of the invention, there can be provided anelectric motor or an electric generator including the coil mountingsystem described above.

According to another aspect of the invention, there can be provided avehicle including a motor of the kind described above.

According to a further aspect of the invention, there can be provided amethod of manufacturing the electric motor or generator described above.The method includes winding a coil for the electric motor or generatoronto the coil tooth. The method also includes attaching the coil toothwith coil to the back portion.

The method can also include connecting the coil to a control deviceconfigured to provide individual current control for the coil. Thismethod lends itself to a motor, which includes control devices of thiskind, since it is not necessary to use a single conductor to produce thewindings for each tooth. Instead, the coil of each tooth can be woundseparately and then connected directly to a control device.

Traction Control

According to another aspect of the invention, there can be provided atraction control system for a vehicle including a plurality of wheels,each wheel being independently powered by a respective motor. Thecontrol system includes sensors for detecting an acceleration in therotation of each of the wheels. The control system also includes acontrol unit for adjusting a torque applied to each wheel by eachrespective motor in response to detecting a predetermined accelerationin the rotation of one or more of the wheels. The predeterminedacceleration is indicative of a skid.

A respective control unit can be provided for each wheel. Each controlunit may be operable to perform traction control independently of othercontrol units in the system according to predetermined rules. Thecontrol units can be networked for exchanging wheel acceleration data.The control units can be operable to provide continuous torqueadjustment for the wheels.

According to a further aspect of the invention, there can be provided avehicle including a plurality of wheels, each wheel being independentlypowered by a respective motor. The vehicle includes the traction controlsystem described above.

Suspension Control

According to another aspect of the invention, there can be provided asuspension control system for a vehicle having a plurality of wheels,each wheel being mounted on a suspension arm of the vehicle and beingindependently powered by a respective motor. The system includes acontrol unit for selectively adjusting a torque applied to each wheel toapply a force to each respective suspension arm.

The control unit can be operable to selectively adjust a torque appliedto each wheel to apply a force to each respective suspension arm toalter a height of the vehicle.

According to a further aspect of the invention, there can be providedvehicle having a plurality of wheels, each wheel being mounted on asuspension arm and being independently powered by a respective motor.The vehicle includes the suspension control system described above.

The motors of the vehicles described above can be electric motors, suchas the electric motors described above.

According to another aspect of the invention, there can be provided atraction control method for a vehicle comprising a plurality of wheels,each wheel being powered by a separate motor. The method includesdetecting an acceleration in the rotation of one or more of the wheels.The method also includes adjusting a torque applied to each wheel byeach respective motor in response to detecting a predeterminedacceleration in the rotation of one or more of the wheels. Thepredetermined acceleration is indicative of a skid.

The predetermined acceleration can be calculated according to adetermined upper limit on the acceleration of the vehicle.

A respective control unit can be used to adjust the torque applied toeach wheel. Each control unit may perform traction control independentlyof other control units in the system according to predetermined rules.Wheel acceleration data can be exchanged between the control units.Continuous torque adjustment can be provided for the wheels.

According to a further aspect of the invention, there can be provided asuspension control method for a vehicle comprising a plurality ofwheels, each wheel being mounted on a suspension arm of the vehicle andbeing independently powered by a respective motor. The method includesselectively adjusting a torque applied to each wheel to apply a force toeach respective suspension arm.

According to another aspect of the invention, there can be provided acomputer program for performing the traction control method and/or thesuspension control method described above.

A computer program for implementing the invention can be in the form ofa computer program on a carrier medium. The carrier medium could be astorage medium, such as a solid state, magnetic, optical,magneto-optical or other storage medium. The carrier medium could be atransmission medium such as broadcast, telephonic, computer network,wired, wireless, electrical, electromagnetic, optical or indeed anyother transmission medium.

Adjusting Seal

Another aspect of the invention is a motor arrangement comprising astator and a rotor, the stator having a stator housing and the rotorhaving a rotor housing, the rotor housing substantially surroundingcomponents of the stator and the rotor having a seal arrangementdisposed between the rotor housing and the stator housing configuredsuch that a member or members of the seal arrangement is moveable from aposition touching the stator housing to a position away from the statorhousing due to centrifugal force on rotation of the rotor housing. Thisarrangement provides the advantage that, when the rotor is stationary orrotating at low speeds, the seal arrangement encloses a gap between therotor and stator, but when rotating at higher speeds, the seal does notwear out by friction between the seal moving with the rotor and rubbingagainst the stator housing. At high speeds, ingress of material into thehousing is prevented by the centrifugal effect of the rotor. Thisprocess is a progressive one in that the pressure between the moveableelement or elements of the seal attached to the rotor and the statorhousing is highest when stationary and reduces as the rotor speedincreases to a level where contact ceases.

The centrifugal action of the rotor also creates a pressure differenceinternally within the rotor stator assembly. This pressure difference isradial and has low pressure at the centre and higher pressureprogressively radially. By incorporation of a suitably protected inletorifice located near the centre of the stator, air is allowed to bedrawn in and subsequently exit at the seal to stator interface. Thismechanism provides for an air film that further protects the seal fromexcessive wear and also provides an additional sealing benefit in thatthe exiting air precludes material ingress. This feature also providesfor the elimination of any water that has entered the motor for exampleas a result of condensation.

Suitably protected inlet orifice can be for example an orifice with apipe attached external to the stator. The pipe is long enough to haveits other open end located in a position which is certain never to beimmersed in water. The open end would further be provided with particlefilter to prevent particles of matter greater than a safe size to enterthe motor.

Another method of protecting the inlet orifice would be by the use of asemi permeable membrane. Such a membrane would allow air to penetratewithout allowing water or particles through (“Goretex” for example).This method can be located at the stator or remotely via a pipe asabove.

Cooling Arrangement

A further aspect of the invention is a motor comprising a coolingarrangement. The motor includes a plurality of coils arranged around acircumference and a cooling channel disposed immediately adjacent theplurality of coils through which a coolant fluid may circulate by beingpumped or by convective flow. This aspect uses a multi faceted coolingplate, which encloses the windings on three sides and provides faces forthe attachment of electronic power devices, a dump power device and adump resistor. The stator assembly comprising the coils, teeth and backiron is assembled directly onto the cooling plate. The assembly is thenpotted onto the cooling plate using thermally conductive material, suchas epoxy filled with aluminium oxide or aluminium nitride or carbon, forexample. This potting process is important due to the mechanicalintegrity imparted to the whole assembly, all parts are as one and moreable to “withstand vibration and shock. The potting further improves theelectrical strength of the insulation system in that it prevents any airpockets within the winding system. Because of the high switching speedsdv/dt is high and this induces electrical stress in the insulationmedium of the windings. Air pockets would risk ionisation and lead toearly failure of the insulation. In electronically controlled motors orgenerators this insulation breakdown brought on by the repeatedelectrical stress induced through the switching events is a majorreliability issue, the potting reduces this risk by a very large degree.Potting is best done under vacuum, but low viscosity potting materialcan be used in atmospheric pressure. The potting is of importance inimproving the thermal conductivity between the heat generating windingsand laminations of the back iron and the heat sink cooling plate withit's cooling fluid inside. The potting is further of great benefit inthat it allows the winding system to be fully immersed in water with norisk of electrical failure. This is important due to the need to makethe electrical system immune to condensation or other water ingress.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention and to show how the same maybe carried into effect reference is now made by way of example to theaccompanying drawings in which:

FIG. 1 schematically shows an example arrangement for a three phasemotor;

FIG. 2 schematically shows the arrangement of coils in one of the coilsub-sets shown in FIG. 1;

FIG. 3 is an exploded view of a motor embodying the invention;

FIG. 4 is an exploded view of the motor of FIG. 3 from an alternativeangle;

FIG. 5 schematically shows an example coil arrangement for a three phasemotor according to an embodiment of this invention;

FIG. 6 schematically shows an example arrangement of coils in one of thecoil sub-sets shown in FIG. 3 according to an embodiment of theinvention;

FIG. 7 schematically shows schematically shows an example arrangementfor a three phase motor according to an embodiment of this invention;

FIG. 8 schematically shows an example arrangement of coils in one of thecoil sub-sets shown in FIG. 7 according to an embodiment of theinvention;

FIG. 9 schematically shows the coils of the embodiment in relation tothe magnets;

FIG. 10 schematically shows an example of a control device in accordancewith an embodiment of this invention;

FIG. 11 is a circuit diagram of the switching arrangement;

FIG. 12 schematically shows an arrangement in which a common controldevice is used to coordinate the operation of a plurality of controldevices;

FIG. 13 schematically shows a front view of a coil tooth in accordancewith an embodiment of the invention;

FIG. 14 schematically shows a side view of a coil tooth in accordancewith an embodiment of the invention;

FIG. 15 schematically shows a back portion with a number of coil teethreceivably attached thereto in accordance with an embodiment of theinvention;

FIG. 16 schematically shows a detailed view of a back portion and coiltooth in accordance with an embodiment of the invention;

FIG. 17 schematically shows a detailed view of a back portion and aplurality of coil teeth in accordance with an embodiment of theinvention;

FIG. 18 schematically shows a detailed view of a back portion and coiltooth in accordance with an embodiment of the invention;

FIG. 19 schematically show examples of a plurality of interconnectedback portions stacked in a staggered formation to form a larger backportion;

FIG. 20 schematically show examples of a plurality of interconnectedback portions stacked in a staggered formation to form a larger backportion;

FIG. 21 shows a seal arrangement;

FIG. 22 schematically shows an example of a vehicle having four wheels,and indicates the forces which are incident upon those wheels;

FIG. 23 schematically shows an example of a wheel mounted on asuspension arm;

FIG. 24 schematically shows the magnet mounting arrangement of anembodiment of the invention; and

FIG. 25 schematically shows a cooling arrangement for the stator coilsof an embodiment of the invention.

DETAILED DESCRIPTION

The embodiment of the invention described is an electric motor for usein a wheel of a vehicle. The motor is of the type having a set of coilsbeing part of the stator for attachment to a vehicle, radiallysurrounded by a rotor carrying a set of magnets for attachment to awheel. For the avoidance of doubt, the various aspects of the inventionare equally applicable to an electric generator having the samearrangement. In addition, some of the aspects of the invention areapplicable to an arrangement having the rotor centrally mounted withinradially surrounding coils.

Physical Arrangement

The physical arrangement of the embodying assembly is best understoodwith respect to FIGS. 3 and 4. The assembly can be described as a motorwith built in electronics and bearing, or could also be described as ahub motor or hub drive as it is built to accommodate a separate wheel.

Referring first to FIG. 3, the assembly comprises a stator 252comprising a rear portion 230 forming a first part of the housing of theassembly, and a heat sink and drive arrangement 231 comprising multiplecoils and electronics to drive the coils as well as a heat sink. Thecoil drive arrangement 231 is fixed to the rear portion 230 to form thestator 252 which may then be fixed to a vehicle and does not rotateduring use. The coils themselves are formed on tooth laminations 235which together with the drive arrangement 231 and rear portion 230 formthe stator 252.

A rotor 240 comprises a front portion 220 and a cylindrical portion 221forming a cover, which substantially surrounds the stator 252. The rotorincludes a plurality of magnets 242 arranged around the inside of thecylindrical portion 221. The magnets are thus in close proximity to thecoils on the assembly 231 so that magnetic fields generated by the coilsin the assembly 231 generate a force on the magnets 242 arranged aroundthe inside of the cylindrical portion 221 of the rotor 240 therebycausing the rotor 240 to rotate.

The rotor 240 is attached to the stator 252 by a bearing block 223. Thebearing block 223 can be a standard bearing block as would be used in avehicle to which this motor assembly is to be fitted. The bearing blockcomprises two parts, a first part fixed to the stator and a second partfixed to the rotor. The bearing block is fixed to a central portion 233of the wall 230 of the stator 252 and also to a central portion 225 ofthe housing wall 220 of the rotor 240. The rotor 240 is thusrotationally fixed to the vehicle with which it is to be used via thebearing block 223 at the central portion 225 of the rotor 240. This hasa significant advantage in that a wheel rim and tyre can then be fixedto the rotor 240 at the central portion 225 using the normal wheel boltsto fix the wheel rim to the central portion of the rotor andconsequently firmly onto the rotatable side of the bearing block 223.The wheel bolts may be fitted through the central portion 225 of therotor through into the bearing block itself. A first advantage of thisarrangement is that the whole assembly may be simply retrofitted to anexisting vehicle by removing the wheel, bearing block and any othercomponents such as the braking arrangement. The existing bearing blockcan then fitted inside the assembly and the whole arrangement fitted tothe vehicle on the stator side and the normal rim and wheel fitted tothe rotor so that the rim and wheel surrounds the whole motor assembly.Accordingly, retrofitting to existing vehicles becomes very simple.

A second advantage is that there are no forces for supporting thevehicle on the outside of the rotor 240, particularly on thecircumferential wall 221 carrying the magnets on the insidecircumference. This is because the forces for carrying the vehicle aretransmitted directly from the suspension fixed to one side of thebearing block (via the central portion of the stator wall) to thecentral portion of the wheel surrounding the rotor fixed to the otherside of the bearing block (via the central portion of the rotor wall).This means that the circumferential wall 221 of the rotor is not subjectto any forces that could deform the wall thereby causing misalignment ofthe magnets. No complicated bearing arrangement is needed to maintainalignment of the circumferential rotor wall.

The rotor also includes a focussing ring and magnets 227 for positionsensing discussed later.

FIG. 4 shows an exploded view of the same assembly as FIG. 3 from theopposite side showing the stator 252 comprising the rear stator wall 230and coil and electronics assembly 231. The rotor 240 comprises the outerrotor wall 220 and circumferential wall 221 within which magnets 242 arecircumferentially arranged. As previously described, the stator 252 isconnected to the rotor 240 via the bearing block at the central portionsof the rotor and stator walls.

Additionally shown in FIG. 3 are circuit boards 80 carrying controlelectronics described later. Due to their kite shape these circuitboards can be referred to as kite boards. Additionally in FIGS. 3 and 4a V shaped seal 350 is provided between the circumferential wall 221 ofthe rotor and the outer edge of the stator housing 230, again describedin detail later. Further, in FIG. 4, a magnetic ring 227 comprising acommutation focusing ring and a plurality of magnets is provided for thepurpose of indicating the position of the rotor with respect to thestator to a series of sensors arranged on the circuit boards 80 of thestator 252. This is also described in greater detail later.

Coil Control

FIG. 5 schematically shows an example of an electric motor in accordancewith an embodiment of this invention. In this example, the motor isgenerally circular. However, it will be appreciated that embodiments ofthis invention can employ other topologies. For example a lineararrangement of coils for producing linear movement is envisaged.

The motor 40 in this example is a three phase motor. Again, it will beappreciated that motors according to this invention can include anarbitrary number of phases (N=1, 2, 3 . . . ). Being a three phasemotor, the motor 40 includes three coil sets. In this example, each coilset includes two coil sub-sets. The coil sub-sets of each coil set arelabelled 44, 46 and 48, respectively. The coil sub-sets 44, 46 and 48are arranged around a periphery of the motor 40. In this example, eachcoil sub-set is positioned opposite the other coil sub-set in that coilset, although such an arrangement is not strictly essential to theworking of the invention. Each coil sub-set includes one or more coils,as described below in relation to FIG. 6.

The motor 40 can include a rotor (not shown in FIG. 5) positioned in thecentre of the circle defined by the positioning of the various coils ofthe motor, thereby to allow rotation of the rotor within the rotatingmagnetic field produced by the coils. Preferably, though, the rotor isarranged around the coils as previously disclosed in FIGS. 3 and 4. Therotor may typically comprise one or more permanent magnets arranged torotate such that their poles sweep across the ends of the coils of themotor 40. Appropriate switching of currents in the coils of the coilsub-sets allows synchronized attraction and repulsion of the poles ofthe permanent magnet of the rotor to produce the rotating action of themotor 40. It will be appreciated that FIG. 5 is highly schematic and, inpractice, the coil subsets will be arranged at the outer periphery ofthe stator with the rotor magnets surrounding the coils.

Each coil set 44, 46, 48 includes one or more coils. As shown in FIG. 6,in the present example, there is a single coil per coil sub-set. Anexample with more than one coil per coil sub-set is described below inrelation to FIGS. 7 and 8. Where more than one coil is provided in agiven coil sub-set, these coils can generally be wound in oppositedirections such that the magnetic field produced by each coil is in ananti-parallel configuration with respect to the magnetic field in anadjacent coil. As described above, appropriate switching of the currentin the coils causes the permanent magnets of the rotor to rotate.

As shown in FIG. 5, in accordance with an embodiment of this invention,the coil or coils of each coil sub-set can be connected to a separatecontrol device 80. In FIG. 5, it is schematically shown that each coilsub-set is connected to the terminals 54, 56, 58 of respective controldevices 80. Accordingly, the coils of corresponding coil sub-sets withina given coil set are not connected in series. Instead, each coil sub-setis individually controlled and powered. The connections to the controldevice and the coils of each coil sub-set can be formed using, forexample, a single piece of wire (e.g. copper wire) as is shownschematically in FIG. 6. There are numerous advantageous to providingindividual power control for the coils of each coil sub-set.

Since there is no need to run connecting wires around the periphery ofthe motor providing series interconnections for the coils of each coilsub-set, less wire is used in manufacturing the motor. This reducesmanufacturing costs as well as reducing the complexity of the motorconstruction. The reduction in wire also reduces conduction losses.

By providing individual power control for the coils of each coilsub-set, and by using a larger number of turns per coil than would beachievable using a motor in which the coils of each coil sub-set areconnected in series, the total inductance of the motor can be greatlyincreased. In turn, this allows far lower current to be passed througheach coil sub-set whereby switching devices having a lower power ratingcan be used for current control. Accordingly, switching devices whichare, cheaper, lighter and less bulky can be used to operate the motor.

The use of lower currents also reduces heat dissipation problems andlowers switching losses due to the faster speed of the smaller switchingdevices which can be employed. The fact that smaller switching devicescan operate at higher frequencies allows for finer and more responsivemotor control. Indeed, torque adjustment can take place on the basis ina highly responsive manner, with adjustments being able to be madewithin a single PWM period. A typical PWM period according to anembodiment of the invention is approximately 50 μs.

Another advantage of the use of smaller switching devices is that theycan be located proximal the coils which they control. In prior electricmotors, where relatively large switching devices have been employed tocontrol the operation of coil sub-sets connected in series, the controldevice is sufficiently large that it can not be included with the othermotor components (e.g. stator, rotor, etc.) but instead has beenprovided separately. In contrast, since small switching devices can beused, in accordance with an embodiment of this invention the switchingdevices and the control devices in which those switching devices areincorporated can be located in, for example the same housing/casing asthe other motor components. Further detail regarding an example of acontrol device incorporating switching devices is given below inrelation to FIGS. 10 and 11.

FIGS. 7 and 8 show another example arrangement for a motor 40 inaccordance with an embodiment of this invention. The motor 40 shown inFIG. 5 is a three phase motor. The motor therefore has three coil sets.In this example, each coil set includes eight coil sub-sets. The coilsub-sets of each coil set are labelled 44, 46 and 48, respectively inFIG. 7. In common with the example described above in relation to FIG.5, each coil set includes pairs of coil sub-sets which are arrangedopposite each other around the periphery of the motor 40. Again,however, it should be noted that there is no express need for each coilsub-set to have a corresponding coil sub-set located opposite from it onthe opposite side of the periphery of the motor 40.

As described above in relation to FIG. 7, each coil sub-set can beconnected to a respective control device 80. The terminals for each coilsub-set of each coil set are labelled 54, 56 and 58, respectively inFIG. 7. While the arrangement shown in FIG. 7 includes a larger numberof coil sub-sets than, for example, the arrangement shown in FIG. 3,this does not significantly increase the size and bulk of the switchingmeans which are used to operate the motor as would be the case if theincreased number of coil sub-sets were connected together in series.Instead, it is merely necessary to provide an additional control device80 incorporating relatively small switching devices as described abovefor each additional coil sub-set. As described above, these controldevices 80 are sufficiently small such that they can be located adjacentto their corresponding coil sub-sets within, for example, the samecasing as the motor 40.

As described above, each coil sub-set can include one or more coils. Inthis example, each coil sub-set includes three coils as is shownschematically in FIG. 8. In FIG. 8, these three coils are labelled 74A,74B and 74C. The three coils 74A, 74B and 74C are alternately wound suchthat each coil produces a magnetic field which is anti-parallel with itsadjacent coils for a given direction of current flow. As describedabove, as the permanent magnets of the rotor of the motor 40 sweepacross the ends of the coils 74A, 74B and 74C, appropriate switching ofthe currents in the coils can be used to create the desired forces forproviding an impulse to the rotor. As is shown schematically in FIG. 6,each coil in a coil sub-set can be wound in series.

The reason that the coils 74A, 74B and 74C within each subset are woundin opposite directions to give antiparallel magnetic fields can beunderstood with respect to FIG. 9 which shows the arrangement of themagnets 242 on the rotor surrounding the coils 44, 46 and 48 of thestator. For simplicity, the arrangement is shown as a linear arrangementof magnets and coils, but it will be understood that in the embodimentof the invention described the coils will be arranged around theperiphery of the stator with the magnets arranged around the inside ofthe circumference of the rotor, as already described.

The magnets 242 are arranged with alternate magnetic polarity towardsthe coil subsets 44, 46 and 48. Each subset of three coils 74A, 74B and74C thus presents alternate magnetic fields to the alternate pole facesof the magnets. Thus, when the left-hand coil of a subset has arepelling force against a North Pole of one of the magnets, the adjacentcentral coil will have a repelling force against a South Pole of themagnets and so on.

As shown schematically in FIG. 9, the ratio of magnets to coils is eightmagnets to nine coils. The advantage of this arrangement is that themagnets and coils will never perfectly align. If such perfect alignmentoccurred, then the motor could rest in a position in which no forcescould be applied between the coils and the magnets to give a cleardirection as to which sense the motor should turn. By arranging for adifferent number of coils and magnets around the motor, there wouldalways be a resultant force in a particular direction whatever positionthe rotor and motor come to rest.

A particular benefit of the independent control of the coil subsets bythe separate control devices is that a larger than normal number ofphases can be arranged. For example, rather than a three phase motor, asdescribed in FIG. 7, higher numbers of phases such as twenty-four phaseor thirty-six phase are possible with different numbers of magnets andcoils. Ratios of coils to magnets, such as eighteen coils to sixteenmagnets, thirty-six coils to thirty-two magnets and so on, are perfectlypossible. Indeed, the preferred arrangement, as shown in FIGS. 3 and 4is to provide 24 separate control “kite” boards 80, each controllingthree coils in a sub-set. Thereby providing a twenty-four phase motor.The use of a multiphase arrangement, such as twenty-four phases,provides a number of advantages. The individual coils within eachsub-set can have a larger inductance than arrangements with lowernumbers of phases because each control circuit does not have to controllarge numbers of coils (which would require controlling a largeaggregate inductance). A high number of phases also provides for lowerlevels of ripple current. By this it is meant that the profile of thecurrent required to operate the motor undulates substantially less thanthe profile from, say a three-phase motor. Accordingly, lower levels ofcapacitance are also needed inside the motor. The high number of phasesalso minimize the potential for high voltage transients resulting fromthe need to transfer large currents quickly through the supply line. Asthe ripple is lower, the impact of the supply cabling inductance islower and hence there is a reduction in voltage transient levels. Whenused in a braking arrangement (described later), this is a majoradvantage, as in hard braking conditions, several hundred kilowatts needto be transferred over several seconds and the multiphase arrangementreduces the risk of high voltage transients in this situation.

The relative arrangement of magnets and coils, shown in FIG. 9 can berepeated twice, three times, four times or indeed as many times asappropriate around 360 mechanical degrees of the rotor and statorarrangement. The larger the number of separate sub-sets of coils withindependent phases, the lower the likelihood of high voltage transientsor significant voltage ripple.

In accordance with an embodiment of this invention, a plurality of coilsubsets with individual power control can be positioned adjacent eachother in the motor. In one such example, three coils such as those shownin FIG. 8 could be provided adjacent each other in a motor but would notbe connected in series to the same control device 80. Instead, each coilwould have its on control device 80.

Where individual power control is provided for each coil sub-set, theassociated control devices can be operated to run the motor at a reducedpower rating. This can be done, for example, by powering down the coilsof a selection of the coil sub-sets.

By way of example, in FIG. 7 some of the coil sub-sets are highlightedwith a ‘*’. If these coil sub-sets were to be powered down, the motorwould still be able to operate, albeit with reduced performance. In thisway, the power output of the motor can be adjusted in accordance withthe requirements of a given application. In one example, where the motoris used in a vehicle such as a car, powering down of some of the coilsub-sets can be used to adjust the performance of the car. In theexample shown in FIG. 7, if each of the coil subsets indicated with an‘*’ were powered down, the remaining coil sub-sets would result in aconfiguration similar to that shown in FIG. 5, although of course thereare three coils per coil sub-set as opposed to the single coil per coilsub-set shown in FIG. 5.

Powering down of one or more of the coil sub-sets has the furtherbenefit that in the event of a failure of one of the coil sub-sets,other coil sub-sets in the motor 40 can be powered down resulting incontinued operation of the motor 40 in a manner which retains a balancedmagnetic field profile around the periphery of the motor for appropriatemultiphase operation. In contrast, in prior systems involving seriesinterconnection of the coils of the coil sub-sets, a failure in thecoils or interconnections associated with any given coil set is likelyto be catastrophic and highly dangerous, given the large currentsinvolved. Moreover, a failure anywhere within the coils orinterconnections between the coils of a given coil set would result inthe motor not being able to continue functioning in any way whatsoever.

In summary, individual power control for the coil sub-sets in accordancewith an embodiment of this invention allows independent powering up andor powering down of selected coil sub-sets in order to react todiffering powering requirements and/or malfunctions or failures withinthe coil sub-sets.

Control Circuitry

FIG. 10 shows an example of a control device 80 in accordance with anembodiment of this invention. As described above, the control device 80includes a number of switches which may typically comprise one or moresemiconductor devices. The control device 80 shown in FIG. 10 includes aprinted circuit board 82 upon which a number of components are mounted.The circuit board 82 includes means for fixing the control device 80within the motor, for example, adjacent to the coil sub-set which it thecontrols—directly to the cooling plate. In the illustrated example,these means include apertures 84 through which screws or suchlike canpass. In this example, the printed circuit board is substantiallywedge-shaped. This shape allows multiple control device 80 to be locatedadjacent each other within the motor, forming a fan-like arrangement.

Mounted on the printed circuit board 82 of the control device 80 therecan be provided terminals 86 for receiving wires to send and receivesignals from a 92 control device as described below.

In the example shown in FIG. 10, the control device 80 includes a numberof switches 88. The switches can include semiconductor devices such asMOSFETs or IGBTs. In the present example, the switches comprise IGBTs.Any suitable known switching circuit can be employed for controlling thecurrent within the coils of the coil sub-set associated with the controldevice 80. One well known example of such a switching circuit is theH-bridge circuit. Such a circuit requires four switching devices such asthose shown in FIG. 10. The wires (e.g. copper wires) of the coilsub-sets can be connected directly to the switching devices 88 asappropriate, and interconnections between the switching devices 88 canbe formed on the printed circuit board 82. Since the switching devices88 can be located adjacent the coil sub-sets as described above,termination of the wires of the coil sub-sets at the switching devices88 is made easier.

As shown in FIG. 11, the control device includes semiconductor switchesarranged in an H-bridge arrangement. The H-bridge is of course known tothose skilled in the art and comprises four separate semiconductorswitches 88 connected to a voltage supply (here 300 volts) and toground. The coils of each sub-coil are connected across the terminals 81and 83. Here a sub-coil 44 is shown connected across the terminals.Simplistically, to operate the motor and supply a voltage in onedirection, switches 88A and 88D are closed and the other switch is leftopen, so that a circuit is made with current in one direction. Tooperate the motor this current direction is changed in harmony with thealternating magnetic polarity passing the coil. To change the directionof rotation of the motor, the timing and polarity of the current flow inthe coil is changed to cause the resulting forces in the oppositedirection. The direction of current flow in the coil is reversed whenswitches 88B and 88C are closed and the other two switches are leftopen. In practice, the technique of pulse width modulating is used topulse width modulate the signal applied to the gate of the semiconductorswitches to control the voltage applied to the coils. The brakingarrangement operates in a manner not known in the prior art and will bedescribed after describing the overall control arrangement.

As shown in FIG. 12, a common control device 92 can be used tocoordinate the operations of the multiple control devices 80 provided inthe motor. In prior motors, in which synchronization of the magneticfields produced by the coils of each coil sub-set is automaticallyachieved by virtue of the fact that they are connected in series.However, where separate power control is provided for each coil sub-set,automatic synchronization of this kind does not occur. Accordingly, inaccordance with an embodiment of this invention, a common control device92 such as that shown in FIG. 12 can be provided to ensure correctemulation of a polyphase system incorporating series-connected coils. Asdescribed above in relation to FIG. 11, terminals 86 can be provided atthe multiple control devices 80 to allow interconnections 90 to beformed between the multiple control devices 80 and the common controldevice 92.

The interconnections 90 can pass signals between the common controldevice 92 and the control devices 80 such as timing/synchronizationsignals for appropriate emulation of a polyphase series-connectedsystem.

In an alternative embodiment, each control unit can operateindependently, without the need of a central control device. Forexample, each control unit can have independent sensors to detect aposition of a rotor of the motor, which would dispense with the need toprovide synchronisation signals of the kind described above. Instead,each control unit would receive a demand signal enabling it to controlthe voltage applied to its associated coils in isolation.

It is stressed that the preferred embodiment does not require any formof central control device for the operation of each wheel incorporatinga motor. Preferably, each motor is self-contained and, within eachmotor, the control circuits 80 are self-contained and depend uponnothing other than a torque demand signal to operate. This means thatthe elements are able to continue to function and to deliver demandedtorque levels, irrespective of any other failures within the total drivesystem. In a system incorporating a plurality of wheels each having amotor, each motor incorporates all the intelligence needed to manage itsactions. Each motor understands its position on the vehicle and controlsits actions accordingly. Preferably, each motor is further provided withinformation regarding the other motors such as the speed, torque andstatus and are based on each motor's knowledge of its position on thevehicle and the state and status of the other motors it can determinethe optimum level of torque that it should apply for a given demandedtorque. Even without this other information, though, the motor cancontinue to respond to a demanded torque.

Other control signals such as power up/power down control signals canalso be sent/received via the interconnects. These signals can alsoinclude signals for adjusting/defining the voltage pulses applied by thecontrol device 80 to the coils of its associated coil sub-set forpowering the motor.

For example, in accordance with an embodiment of this invention, meanscan be provided for monitoring a back EMF within the coil or coils of acoil subset. The task of emulating a motor with series connected coilsub-sets as described above is complicated by virtue of the back EMFassociated with the motor. In a series connected system, the back EMFsare also in series and this gives rise to a smooth sine wave back EMFprofile. Accordingly, in a series configuration the sinusoidal back EMFminimises the bandwidth required from the drive electronics whencontrolling the current in the coils.

In contrast, the reduced number of coil sub-sets connected in series inaccordance with an embodiment of this invention can result in a nonsinusoidal back EMF. Accordingly a more agile control system isdesirable in order to ensure that the currents in the coils remainsinusoidal.

According to an embodiment of this invention, near instantaneouscompensation can be provided for back EMF and further adjusting for anyvariations in a system dc supply voltage. The means for measuring theback EMF can include a current sense device fitted to provide feedbackof the actual current flowing in the coil or coils of each coil sub-set.In one example, a simple series resistor of suitably low value in serieswith the switching devices can be employed. For example, in oneembodiment two resistors can be provided in the bottom emitter of a “H”bridge power stage.

As the back EMF changes with rotor angle and rotor velocity, thisresults in a change in the rate of change of current in the coil. Thisrate of change of current can be detected across a resistor or othercurrent sense device as a change in voltage. This change can then bedifferentiated to produce a voltage which is proportional to the backEMF.

Similarly, the supply voltage can be applied to a capacitor at the startof each PWM period. The resulting voltage ramp can be added to the backEMF signal and combined as a feed forward term to modify the current PWMperiod up or down. Thus both supply variation and back EMF changessubstantially instantly adjust the PWM period and hence voltage appliedto the coil, resulting in rapid adjustment of coil current to follow thedemanded value.

In a further example, a sense coil can be provided. Sense coils can beprovided around, for example, a sub-set of coil teeth of the kinddescribed below. The sense coil then can be monitored at appropriatetimes for the back EMF voltage. This in turn can be used in a similarmanner as described above to feed forward a term to adjust PWM period inmid-cycle, response to the magnitude of the back EMF.

In embodiments where each drive module generates its own PWM signal,back EMF correction thus can take place in a manner which is notsynchronised with the other modules resulting in a distributed randomspread spectrum. Alternatively the control devices can have their PWMgenerators synchronised by an off board device such as the commoncontrol device 92.

The control device can also optionally include means for monitoring atemperature within the motor, for example within the coils sub-setassociated with that control device 80. The control device can beconfigured automatically to respond to the temperature measurement to,for example, reduce power to the coils sub-set to avoid overheating.Alternatively, the temperature measurement can be passed onto the commoncontrol device 92 from each control device 80, whereby the commoncontrol device 92 can monitor the overall temperature within the motorand adjust the operation of the control devices 80 accordingly.

Noise Reduction

In accordance with an embodiment of the invention, EMI noise can bereduced by providing for staggered switching of the switches within eachcontrol device 80. By including a slight delay between the switching ofthe various switching devices in the motor, a situation can be avoidedin which a large number of switching events occur in a short amount oftime, leading to a peak in EMI noise. Thus, the staggering of theswitching within the switches 88 of the control devices 80 can spreadthe EMI noise associated with the switching events during operation ofthe motor across a wider time period thereby avoiding an EMI noise peak.This kind of spreading of the switching events can be coordinatedlocally at the individual control devices 80 or could alternatively becoordinated by the common control device 92 using adjusted timingsignals sent via the interconnections 90.

Power Supply

Although the control devices 80 described in this application canprovide individual power control for the coils of each coil sub-set in amotor, and although this may be achieved using various kinds ofswitching devices and arrangements, the control device system cells canbe coupled to a common power source such as a DC power supply. Aparticularly useful arrangement for the DC power supply is to provide acircular bus bar. Because the control circuit 80 are arranged in a ring,the DC power feed may also be arranged as a ring. This providesincreased safety in that there is a current path around each side of thering (in the same way as a domestic ring main) and so breakage of the DCsupply at one point will not prevent power reaching the controlcircuits. In addition, because current can flow from the source powersupply to each control circuit by two routes through the circular busbar, the current demand on the bus bar is halved.

Braking Arrangement

A number of the features already described provide a significantadvantage when implemented in a motor within a vehicle wheel inproviding a safe mechanism for applying a braking force and therebyavoid the need for a separate mechanical braking arrangement. The motoritself can provide the braking force and thereby return energy to thepower supply, such that this arrangement may be termed “regenerative”braking. When operating in this mode, the motor is acting as agenerator.

The braking arrangement makes use of the considerable redundancy builtinto the motor assembly as a whole. The fact that each separate coilsub-set 44, shown in FIGS. 7 and 8, is independently controlled by aswitching circuit 80 means that one or more of the switching circuitsmay fail without resulting in a total loss of braking force. In the sameway that the motor is able to operate with reduced power when providinga driving force by intentionally switching some of the switchingcircuits to be inoperable, the motor can operate with a slight reductionin braking force if one or more of the switching circuits fail. Thisredundancy is inherent in the design already described but makes themotor a very effective arrangement for use in a vehicle, as it canreplace both the drive and braking arrangement.

A further reason why the motor assembly can provide an effective brakingarrangement is in relation to the handling of power. As alreadymentioned, the use of multiple independently controlled coils means thatthe current through each coil when operating in a generating mode neednot be as high as the current through an equivalent arrangement withfewer phases. It is, therefore, simpler to deliver the power generatedby the coils back to the power source.

To ensure safe operation of the braking arrangement, even in the eventof failure of the power source, the circuitry 80 for each individualcoil sub-set is itself powered by an electricity supply derived from thewheel itself. As the wheel rotates, it generates a current as themagnets pass the coils. If the power supply fails, this current is usedto supply power to the switches 80.

A further redundancy measure is in providing separate physical sensorsconnected to the brake pedal (or other mechanical brake arrangement) ofthe vehicle, one sensor for each wheel. For example, in a typicalfour-wheeled car, four separate brake sensor arrangements would bephysically coupled to the brake pedal with four separate cables going tothe four separate motors. Accordingly, one or more of these separateelectrical sensors connected to the mechanical brake pedal or, indeed,the separate cables could fail and still one or more of the wheels willbe controlled to operate a braking force. By virtue of the ability ofthe control units to communicate with each other, software featuresallow the failure of any sensor or it's cable to have no effect on themotor operation. This is achieved by each motor being able to arbitratethe sensor information and use the sensor data from the other motors ifit's sensor data is disparate with the other three sensors.

A yet further redundancy measure is the use of a so-called dumpresistor. In the event of failure of the power supply, the energygenerated by the wheel, when providing a braking force, needs to bedissipated. To do this, a resistance is provided through which theelectrical power generated by the wheel may be dissipated as heat. Theuse of the multiphase design with separate electrical switching of eachsub-coil allows the use of distributed resistance, so that each sub-coilmay dissipate its power across a resistance and the dump resistance as awhole may therefore be distributed around the wheel. This ensures thatthe heat thereby generated can be evenly dissipated through the mass ofthe wheel and the cooling arrangement.

Referring again to FIG. 11, the mode of operation of the switch 80 foreach coil sub-set 44 is as follows when in a braking mode. The upperswitches 88A and 88B are opened and switch 88C operated in on/off pwmmode to control the voltage generated by the coil. As the magnet passesthe coil sub-set 44, the voltage at connection point 83 rises. When theswitch 88C is then opened as part of the pwm process, the voltage atpoint 83 rises to maintain the coil current and so energy is returned tothe power supply (via the diode across switch 88B). This arrangementeffectively uses the coils of the motor itself as the inductor in aboost form of DC-to-DC converter. The switching of the controls in the Hbridge circuit controls the DC voltage that is provided back to thepower source.

The boost type dc/dc converter switching strategy employed forregenerative braking has a further distinct advantage in that it reducesbattery loading. In known systems regenerative mode operates byswitching the top switches to provide the battery volts in series withthe motor coil and its back emf. This requires the current to beestablished through the battery. Hence even though the coil isgenerating, it depletes the battery state of charge by virtue of itscurrent having to flow through the battery in the discharge direction.By employing the DC-to DC converter arrangement described above, thecoil establishes its current locally by an effective short circuitacross the coil, created by the bottom switches. When the generatedcurrent is established it is then directed back to the battery in thecharge direction. So whilst both regimes collect the transient energywhen the bottom switch turns off in the normal pwm sequence, theconventional system consumes battery current whilst establishing thegenerated current flow, whereas the arrangement here described consumesno battery current.

When the voltage generated by the coil falls below say four volts, thecurrent can no longer flow due to the voltage dropped across theswitches or diodes used within the H bridge circuit. In the embodiment,a voltage of approximately 1.75 volts per mile per hour is generated andso at speeds below 3 miles per hour, this situation arises. At thisspeed, the switching strategy changes to a form of DC plugging. In DCplugging the phase of all voltages is arranged to be the same. Thiscommon phase of all voltages results in the removal of rotation forceand the application of a static force. The static force attempts to holdthe rotor in one position. Thus normal pwm control is used but with eachcoil subset having it's applied voltage in phase with all others. ThisDC mode of operation is particularly beneficial at low speeds, as itensures safe stopping of the vehicle. When the vehicle has come to acomplete rest, the vehicle will stay at rest, as any movement of therotor is resisted by the static field. There is thus no risk that themotor would accidentally move forwards or backwards.

The dump resistor arrangement already described may also be used in theevent that the battery is simply full and energy needs to be dissipatedwhen braking. If the voltage across the supply goes over a giventhreshold then energy may be switched to the dump resistor.

Embodiments of this invention can provide a highly reliable motor orgenerator, at least in part due the separateness of the power controlfor the coil sub-sets as described above. Accordingly, a motor orgenerator according to this invention is particularly suited toapplications in which a high degree of reliability is required.

A further safety feature, particularly beneficial when incorporated in avehicle, is that the motor can supply power not only to the switcheswithin the motor, but also to remote aspects of a whole system,including a master controller processor, shown as common control device92, in FIG. 12, and to other sensors, such as the break pedal sensor. Inthis way, even if there is a total failure of power supply within thevehicle, the braking arrangement can still operate.

For example, applications such as wind turbines depend for their successand take up on cost and reliability. Typical turbine systems will runfor 25 years and ideally should require minimum in service down time formaintenance/breakdown etc. By incorporating the drive electronics into acompact form with compact windings, as can be achieved according to anembodiment of this invention, the total system cost can be minimised. Inaccordance with an embodiment of this invention, independent powercontrol of the coil sub-sets can allow continued operation even underpartial failure the system.

In particular vertical axis turbines which are recently growing inpopularity due to their efficient operation can benefit from theincorporation of a motor according to this invention. This is because ofthe high power to weight ratio which can be achieved, which allows forlower mast head mass and hence less cost for support column/structure.

Military, marine aircraft and land based drive systems are all currentlyless reliable than would be desired due to the dependence on singledevice reliability in classic 3 phase bridge topologies. Again, using amotor according to an embodiment of this invention, the reliability ofsuch vehicles could be improved.

It will be clear from the foregoing description that electric motorsgenerally include a complex arrangement of interconnections andwindings. As described above, the manufacture of an electric motorincorporating such features is a laborious and time consuming process.The time and effort which is required to construct an electric motor isgenerally exacerbated by the use of, for example, copper wire for thewindings and interconnections. Wire of this kind is often relativelythick (in order to be able to handle high currents) and is difficult tomanipulate. Damage to electric insulation provided on the wire can bedifficult to avoid during motor construction, again due to thedifficulty in manipulating the wire. Access to the relevant parts of amotor for installing the windings and interconnections is often limitedand inhibited by other components of the motor.

Coil Mounting

FIGS. 13 to 18 schematically show an example of a coil mounting systemfor an electric motor or indeed for an electric generator which allowseasier construction and assembly of a motor. As shown in FIG. 15, thecoil mounting system includes a back portion 150 and a plurality of coilteeth 100, which are interspersed around a periphery of coil mountingportion 150. The teeth are positioned such that they can receive thewindings of motor coils to produce the desired N-phase motor. Thearrangement of teeth shown in FIG. 11, for example, could be used toconstruct a three phase electric motor of the kind described above.

The coil teeth 100 are shown in more detail in FIGS. 13 and 14. FIG. 13shows a front view of a coil tooth and FIG. 14 shows a side view of thecoil tooth. The tooth 100 includes an elongate arm 102 for windinglyreceiving a coil of the electric motor. A flange 104 can be provided atone end of the elongate arm 102 of the coil tooth 100 to preventinadvertent unravelling of the coil from the coil tooth once it has bewound thereon.

In accordance with an embodiment of the invention, each coil tooth isattachably receivable on the back portion 150. Accordingly, each coiltooth can include means for attaching the coil tooth 100 to the backportion 150. In the example shown in FIGS. 9 and 10, these means includetwo elongate fingers 106. These fingers extend from an end of theelongate arm 102 opposite the flange 104 in a generally V-shapedconfiguration. As will be discussed below, the back portion 150 can havecorrespondingly shaped portions to receive these fingers for attachingthe coil tooth 102 to the back portion 150.

FIGS. 16 and 17 show the back portion and a received coil tooth in moredetail. In the examples shown in FIGS. 16 to 17, the back portion ismade up of plurality of smaller back portions 150 which are joinedtogether.

FIG. 16 shows a single coil tooth 100 attachably received in the backportion 150. As shown in FIG. 16, the back portion 150 can include aplurality of openings 126 for receiving correspondingly shaped featuresof the coil tooth 100, such as the aforementioned V-shaped fingers 106,for allowing attachment of the tooth 100 to the back portion 150.

FIG. 16 shows an example of how the wire of a motor coil can bewindingly received around the elongate arm 102 of the coil tooth 100 andterminated at terminals 142. The terminal may comprise, for example, theterminals of a control device of the kind described above. Since thetooth 100 is attachably receivable in the back portion 150, the windingsof the coil can be received by the coil tooth 100 at a manufacturingstage prior to attachment of the tooth 100 and the back portion 150.This allows the winding process to be performed away from the rest ofthe motor components, such that they cannot inhibit the winding process.Accordingly, easy access to the elongate arm 102 can be achieved whilethe tooth 100 is separate from the back portion 150, and the coil can bewound with minimal risk to the electrical insulation provided thereon.This allows wires having less electrical installation material to beused, which in turn allows for better heat dissipation per unit lengthof coil. This in turn allows the power rating of the motor comprisingthe coil mounting system to be maximized.

The coil mounting system described herein can be used in conjunctionwith electric motors which employ series-connected coil sub-sets asdescribed in relation to FIG. 1. In such examples, construction of theelectric motor may include taking a single piece of wire and winding itaround a plurality of coil teeth away from the back portion, whileleaving a length of wire between each tooth to provide the appropriateseries interconnections. Once this has been done, the coil tooth couldthen be attached to the back portion in the appropriate positions.

The coil mounting system described herein lends itself in particular,however, to a motor in which the coil sub-sets are individuallycontrolled. This is because the individual controls require separatetermination of the coil teeth of each coil sub-set, whereby it is notnecessary to judge the appropriate length of plurality of seriesinterconnections between each coil sub-set when winding the wires aroundthe coil teeth away from the back portion. Instead, a plurality of coilteeth and associated windings can be produced in a first step and thenthose coil teeth can simply be arranged as desired on the back portion150.

As described above, there can be more than one coil per coil sub-set.FIG. 17 shows that the present coil mounting system is also compatiblewith such coil sub-sets. In particular, FIG. 17 shows an example of howthree coil teeth 100 can be wound and deployed to form the coils of acoil sub-set according to the present coil mounting system. Thearrangement of the windings shown in FIG. 17 corresponds to thearrangement of the coils shown in FIG. 8 as described above.

Further details of the back portion 150 will now be described inrelation to FIG. 18. The back portion 150 shown in the figures issubstantially arcuate. This allows a plurality of back portions to beinterconnected to form a circular larger back portion thereby toconstruct a circular motor. The back portions 150 are interconnectableto form the larger back portion. In the example shown in FIG. 18 theback portions can be interconnected using features of the coil tooth100. In particular, it can be seen that the coil tooth 100 shown in FIG.18 joins together two back portions 150, with one of its fingers 106received in receiving opening 126 of each of the back portions 150. Inother examples, alternative means of interconnecting the back portionscan be provided. It is also envisaged that a circular back portion 150can be provided having a single-piece construction.

Where multiple back portions 150 are interconnected to form a largerback portion, via one of the coil teeth 100, it should be noted that thejoin between adjacent back portions 150 run substantially parallel tothe magnetic field produced by a coil windingly received on the coiltooth 100, whereby the join does not substantially interfere with themagnetic field produced by the coil.

As shown in FIG. 18, raised portions 128 can be provided on the outerperiphery of the back portion 150 to conform with the shape of the coilteeth.

The back portion 150 can also be provided with features for aiding inheat dissipation. In the present example, the back portion 150 isprovided with cut away portions 130. These cut away portions serve toprovide a lighter construction for the back portion 150, which istypically constructed from steel or another metal. In accordance with anembodiment of this invention, one or more pins 134 which can bemanufactured from, for example aluminium, can be inserted into the cutaway portions 130 for providing an improved thermal contact between theback portion 150 and another component of the motor such as the stator.

As shown in FIG. 18, the back portion 150 can have a laminatedconstruction comprising a plurality of layers 115. The back portion canbe manufactured using a stamping process in which the plurality oflayers 115 are stamped according to the desired shape and configurationof the back portion, stacked together as shown in FIG. 18, and thenjoined together to form the back portion 150.

This coil mounting system comprising a plurality of interconnected backportions 150 allows an efficient stamping process to be used forconstructing the teeth and/or back portions 150 using a laminatedconstruction. This is because relatively little material is lost orwasted, during the stamping process, in contrast to the case where aback portion comprising a single piece laminated construction withoutinterconnections 126 is stamped.

As described above, a plurality of back portions 150 can beinterconnected to form a larger back portion. Moreover, a plurality oflayers of such interconnected back portions can be stacked together toform a still larger back portion. Examples of this are shown in FIGS. 19and 20. FIG. 19 and

20 show a side view of the stacked back portions and there correspondingjoins

126. As is shown in FIGS. 19 and 20, the joins 126 can be staggered in anumber of different configurations such that adjacent layers ofinterconnected back portions 150 do not have interconnections 126 whichcoincide with each other vertically. This serves to increase thestrength of the larger back portion comprising the stacked,interconnected back portions 150.

An advantage of using a large number of coils, in conjunction with thearrangement shown in FIGS. 13 to 17, relates to the thickness of theback iron portion 150 needed for a given application. Typically, thethickness of the back portion 150 is approximately one quarter to onehalf the magnet width to cope with the magnetic flux. Fewer coils wouldrequire larger magnets and, therefore, more iron mass leading to aheavier motor design. By using a large number of separate coils, thethickness of the back iron portion 150 can be reduced. The thickness ofthe iron supporting the magnets on the rotor can also be reduced. It isimportant that the thickness of the rotor, as a whole, can be as thin aspossible, so as to apply the force between the coils and the magnets asclose as possible to the outer rim, thereby increasing the turningmoment provided.

Seal Arrangement

FIGS. 3 and 4 show various views of an example of a motor according toan embodiment of this invention, and the mechanism for sealing theenclosure will now be described. FIGS. 3 and 4 show a front view and aback view of the motor 210, respectively. The motor 210 in this exampleincludes a casing which has a front portion 220 and a rear portion 230.In FIG. 3, the rear portion 230 and a further cover portion have beenremoved to reveal the contents of the casing.

In this example, the motor 210 includes a rotor 240 which rotatesrelative to the stator 252, which may remain stationery during operationof the motor 210. In this example, the rotor 240 includes a plurality ofpermanent magnets 242, which are arranged in a circle within thesubstantially circular rotor. As described above, the rotating magneticfields formed within the motor 210 can provide the necessary attractiveand repulsive forces for producing rotational movement of the rotor 240.

The stator 252 can include an arrangement of sets of coils as describedabove. In particular, in the present example, the coils are arranged incoil subsets, each of which is individually controlled by acorresponding control device 280. These control devices 80 are visiblein FIG. 3. Underneath the front portion 220, a plurality of plates canbe provided to protect to the components of the control devices 280 ofthe motor 210. As described above in relation to FIG. 7, thesecomponents can include one or more semi-conductor devices such asMOSFETs or IGBTs. In the present example, IGBTs are mounted on a printedcircuit 82 board as shown in FIG. 10. In this example, there are fourIGBTs 88 per control device 80.

Each control device 80 can control the coils of a respective coilsub-set as described above. In the present example, the stator 252includes a plurality of coil teeth 200, which are mounted on a backportion 250. The coil teeth 200 and back portion 250 can be of the kinddescribed above in relation to, for example, FIGS. 13 to 20.

Returning to FIG. 3, it can be seen that the front portion 220 of themotor casing is provided with first and second hatches 222. Thesehatches 222 allow access to the control devices 80 of the motor 210 forinstallation, maintenance and/or repair purposes. Means such as screws224 can be provided to allow covers of the hatches 222 to be removablyattached to the front portion 220. By rotating the front portion 220 ofthe motor casing relative to the stator 252 and the various controldevices 80, access can be gained to the appropriate plate of a desiredcontrol device 280. The plate can then be removed to allow access to thecontrol device 80 itself. In this way, the control devices 80 can bemaintained/repaired etc without having to completely remove the frontportion 220 from the motor 210.

FIG. 21 shows a further part of the seal arrangement by which ingress ofmaterial is prevented into the housing of the motor assembly. This showsan enlarged portion of the stator wall 230 and circumferential wall 221of the rotor at the point at which they meet. A small air gapnecessarily exists between the rotor wall and stator wall to allowrotation of one with respect to the other. This gap is filled with a Vseal 350, as shown in FIGS. 3 and 4 and shown in greater details in FIG.21. The seal is fixed to the circumferential wall to one of the rotorhousing and a free end of the seal abuts the wall 230 of the statorhousing. As the rotor rotates with increasing speed, the free end of theseal is caused to deflect, due to centrifugal force, away from the wall230 of the stator, thereby minimizing the wear of the seal 350. Ingressof dirt or other materials into the housing is prevented by thecentrifugal force caused by rotation of the rotor. In addition, this isassisted by allowing a flow of air through a small hole 351 in thestator wall. This allows a flow of air from the small hole across theinterior face of the stator wall 230 and out through the gap presentedby the deflected seal 350, thereby ensuring that ingress of dirt orother material is prevented.

It will be clear from the foregoing that embodiments of this inventionare applicable to electric generators as well as to electric motors, duein part to the structural and conceptual similarity between the two. Forexample, an electric generator can benefit from separate powertermination of the coils of a coils subset as described above.Furthermore, the coil mounting system described above is equallyapplicable to the construction of the arrangement of coils in agenerator and a motor.

Traction Control

As discussed above, motors constructed according to an embodiment ofthis invention can allow for highly responsive torque control.Furthermore, according to an embodiment of this invention, each wheel ofa vehicle can have its own motor. For example, a motor of the kinddescribed above can be provided locally for each wheel.

The use of separate motors for each wheel of a vehicle can allow forincreased flexibility in handling traction control for the vehicle.Moreover, the short response times for torque control afforded by amotor according to an embodiment of this invention can enhance thisflexibility.

In accordance with an embodiment of the invention, each wheel of avehicle can be controlled by its own motor and corresponding drivesoftware, thereby allowing each motor to handle its own tractioncontrol. This means that each motor can handle, for example, a skidsituation independently of the other wheels. Moreover, the fast responsetimes (e.g. within a single PWM period of, for example 50 μs) affordedby embodiments of this invention can allow intricate control of thetorque applied to each wheel independently, for increased effectivenessin handling a skid.

In accordance with an embodiment of this invention, motor control is bya high speed continuous range torque loop. This can allow the responseto be smoother and the achieved grip to be greater than a mechanicallymodulated skid management system. The motor drives can be networkedtogether by a controller area network (CAN). This can allow informationregarding, for example, skid events to exchanged between the motordrives for coordinated action to be taken. In one example, thisinformation includes acceleration data indicative of the angularacceleration of each wheel. A sharp increase in angular acceleration canbe interpreted as a wheel slip of a wheel skid.

Accordingly, an important part of skid management is the detection ofthe onset of a skid event. As a vehicle such as a car has substantialmass and a wheel has a much smaller mass, an upper limit can be placedupon the rate of change of angular velocity of the wheel, above which askid must be occurring. This limit can be predetermined according tofactors including the weights of the vehicle and the wheels.

According to an embodiment of the invention, sensors such as internalmagnetic angle sensors can be provided in the motor of each wheel or inthe wheels themselves. These sensors can detect the angular velocity ofeach wheel. By taking the first derivative of the angular velocitydetermined by the sensors, the angular acceleration of each wheel can bedetermined for wheel skid determination.

In another embodiment, wheel skid can be detected comparing each wheelspeed with that of the other wheels.

As described above, wheel skid could be detected by detecting changes inthe angular velocity of a wheel.

The upper limit for the acceleration of a vehicle can be defined interms of the total power of all motors (P motors) and the vehicle mass(m_(vehicle)):P _(motors) =F _(air) ·v _(vehicle) +m _(vehicle) ·v _(vehicle) ·a_(vehicle)  (1)where v_(vehicle) is the vehicle velocity and F_(air) is a measure ofthe air resistance encountered by the vehicle. Equation (1) can berearranged to form an expression for the upper limit of the accelerationachievable by the vehicle:

$\begin{matrix}{a_{vehicle} \leq \frac{P_{motors} - {F_{air} \cdot v_{vehicle}}}{m_{vehicle} \cdot v_{vehicle}}} & (2)\end{matrix}$

Using equation (2), an expression for an upper feasible limit on theacceleration of a wheel of the vehicle α_(wheel) can be determined:

$\begin{matrix}{{\alpha_{wheel} \leq \frac{a_{vehicle}}{r_{wheel}}} = \frac{P_{motors} - {F_{air} \cdot v_{vehicle}}}{r_{wheel} \cdot \left( {m_{vehicle} \cdot v_{vehicle}} \right)}} & (3)\end{matrix}$

where r_(wheel) is the radius of the wheel. This expression could besimplified by assuming that air resistance is negligible (i.e.F_(air)≈0):

$\begin{matrix}{\alpha_{wheel} \leq \frac{P_{motors}}{r_{wheel} \cdot m_{vehicle} \cdot v_{vehicle}}} & (4)\end{matrix}$

For wheel accelerations above the value defined in either equation (3)or equation (4), it can be concluded that a skid is occurring in thatwheel.

The actual angular acceleration of a wheel can be determined bycomputing the derivatives of angular positions of the wheels withrespect to time.

In accordance with an embodiment of the invention, this could beachieved using a method such as the Newton finite elements method.

In accordance with another embodiment of the invention, this could bedone by interpolating from measurements of angular position of the wheeland then taking derivatives of the interpolated function. In oneexample, a Pronney interpolation can be used. Using this interpolation,the first and second derivatives of a function f(x) can be defined as:

$\begin{matrix}{{{f(x)}^{\prime} = {{\underset{{\Delta\; x}->0}{Lim}\frac{f\left( {x + {\Delta\; x}} \right)}{\Delta\; x}} = {\underset{{\Delta\; x}->0}{Lim}\frac{{f\left( x_{2} \right)} - {f\left( x_{1} \right)}}{x_{2} + x_{1}}}}}{{and}\text{:}}} & (5) \\{{f(x)}^{''} = {\underset{{\Delta\; x}->0}{Lim}\frac{{f\left( {x - {\Delta\; x}} \right)} - {2 \cdot {f(x)}} + {f\left( {x + {\Delta\; x}} \right)}}{\Delta\; x^{2}}}} & (6)\end{matrix}$

Regarding equations (5) and (6) a basic formula for estimation ofangular velocity can be defined as:

$\begin{matrix}{{\omega \approx \frac{\theta_{k} - \theta_{k - 1}}{\Delta\; t} \approx {f \cdot \left( {\theta_{k} - \theta_{k - 1}} \right)}}{{which}\mspace{14mu}{gives}\text{:}}} & (7) \\{\frac{\omega}{f} \approx {\theta_{k} - \theta_{k - 1}}} & (8)\end{matrix}$

where k indexes a k^(th) measurement of the angular position of a wheel.Accordingly, in one example, acceleration estimation can be based atleast on two intervals defined with three points (where each pointconstitutes a k^(th) measurement of angular position):

$\begin{matrix}{\alpha \approx \frac{\omega_{k} - \omega_{k - 1}}{\Delta\; t} \approx \frac{\frac{\theta_{k} - \theta_{k - 1}}{\Delta\; t} - \frac{\theta_{k - 1} - \theta_{k - 2}}{\Delta\; t}}{\Delta\; t} \approx \frac{\theta_{k} - {2 \cdot \theta_{k - 1}} + \theta_{k - 2}}{\left( {\Delta\; t} \right)^{2}}} & (9)\end{matrix}$

As described above, the determined actual acceleration of each wheel canbe compared with the limit defined by, for example, the values given byequations (3) and/or (4) above to determine the occurrence of a skid.

When a skid is detected it is the aim of the motor drive software totake the wheel out of skid as soon as possible. In order to achievethis, the torque applied to the wheel or wheels which are skidding canbe reduced. As the torque is reduced, the angular velocity of the wheelwill decrease until the wheel stops skidding. Accordingly, the task ofthe drive software is to reduce the torque applied to the skidding wheelin a manner which allows the skid to be quickly abated.

There are a number of ways in which the torque applied to a skiddingwheel can be reduced for regaining traction. For example, a combinationof a calculated step reduction in torque followed by a linear reductioncould be applied until it is detected that traction been regained.Alternatively, the torque could be dropped to zero or a very low value.The time taken for the wheel to stabilise back to the average vehiclespeed could then be determined. This would give enough information tofind the grip coefficient of the tyre as the rotational inertia of thewheel is known in advance. In turn, this measurement can then be used tomodulate the torque produced in the wheel motor.

As described above, the motor drives of a vehicle can be networkedtogether by, for example, a controller area network (CAN). Networking ofthis kind can allow the motor drives to communicate for providingimproved awareness of each motor drive as to the overall condition ofthe vehicle. For example, in such a configuration, the motor drives canprovide for the maintenance of left/right traction balance across thefour wheels of, for example, a car. This can allow a significantleft/right imbalance, which could alter the steering direction of thecar or even spin it around, to be corrected for.

With reference to FIG. 22, if one wheel detects skid, pairs of wheels(for example, wheels 312 and 314, or wheels 316, 318) can react to it bytaking similar action. However, a situation should be avoided in whichwheels on opposite sides of the vehicle take action, because this couldcause the rotation of the vehicle by creating torque. An asymmetricforce on different sides of vehicle (left or right one) would producesuch a torque. For example if a vehicle were driving along a soft vergewith the left wheels of the vehicle on grass and the right wheels ontarmac and the driver braked suddenly, then the vehicle would respond byveering to the right—possibly into the path of oncoming traffic.

According to an embodiment of the invention, traction balancing can beemployed which balances the traction of pairs of wheels as describedabove. For example, left/right balancing can be performed within acertain level of buffering to match the torque loadings of the left andright sets of motors. Balancing of this kind may not be necessary at lowspeeds or where no significant amount of braking is taking place. Inanother example, front/rear balancing can be employed—this can allow thevehicle to take maximum advantage the power of each motor.

By way of example, in a vehicle such as car having four wheels, if theleft front wheel is skidding but all the others are not, torque can bereduced to the front left wheel, but increased to the rear left to makeup the difference. However, in some situations, the rear left may not beable to fully make up for the reduction in torque of the front left. Ifthis is the case then both the right hand side motors could reduce theirtorque to even the left/right torque balance to within a certain‘buffer’ amount. Predetermined rules can be employed by each drive motorto determine how it should react in a given situation. Since the rulesare predetermined, each motor can assume that other drive motors in thevehicle will react in a manner which is defined by those rules. In thisway, the drive motors can act in a co-ordinated manner as describedabove. Furthermore, the predetermined rules can be altered according to,for example, driving conditions including the wetness of the roadsurface and the temperature of the road.

Suspension Control

The use of separate motors for each wheel of a vehicle can also allowfor increased flexibility in handling suspension control for thevehicle. Again, the short response time for torque control afforded by amotor according to an embodiment of this invention can enhance thisflexibility still further.

In accordance with an embodiment of the invention, there can be provideda suspension control system for a vehicle having a plurality of wheels.With reference to FIG. 23, each wheel 330 in this example can be mountedon a suspension arm 340.

In FIG. 23, the normal direction of travel of the vehicle with respectto the surface 350 is shown in FIG. 23 by the arrow labelled Z.Accordingly, as shown in FIG. 23, the wheel 330 is rotates in thedirection indicated by the arrow labelled X. If additional torque isapplied to the wheel in the direction indicated by the arrow labelled X,this will tend to impart a force on the suspension arm 340, whereby thesuspension arm 340 will tend to rise up in the direction indicated inFIG. 23 by the arrow labelled Y. In this example, the wheel 330 is afront wheel of a vehicle such as a car. The rising up of the suspensionarm 340 in the direction indicated by the arrow labelled Y in FIG. 23,would therefore cause the front of the car in locality of the wheel 330to rise up also. In this way, by adjusting the torque to the wheel of avehicle, a suspension control system for a vehicle can be implemented.

In accordance with an embodiment of this invention, there can beprovided a suspension control system in which the torque applied to eachwheel of a vehicle can be selectively adjusted for adjusting thesuspension and/or height of the vehicle. The control system can beimplemented by a control unit which can be provided either for eachrespective wheel or which can be provided as a central control unitoperable to control a plurality of wheels. Each wheel can be powered,for example, by an electric motor of the kind described above. In suchembodiments, the fine control and swift response times afforded by themotor described above can enhance the responsiveness of the suspensioncontrol. The control devices for controlling the suspension can benetworked together using a control area network (CAN) as describedabove.

In a first example of a situation in which the ability to independentlycontrol the suspension of a plurality of wheels using torque control iswhere a vehicle is travelling around a corner. When a vehicle istravelling round a corner, it may be desirable to tilt the vehicle suchthat the side of the vehicle on the outside of the corner rises up withrespect to the side of vehicle on the inside of the corner. This can beused to counterbalance the tendency of the outside of the vehicle totilt downwards toward the surface of the road. In this example, moretorque can be provided to the outer wheels of the vehicle (i.e. thewheels of the vehicle on the outside of the corner) in order to raisethat part of the vehicle. Conversely, less torque can be provided to thewheels on the inside of the corner. This would lower the suspension ofthe vehicle on the inside of the corner as desired.

The above example describes a situation in which pairs of wheels in thevehicle are controlled together, to raise, or lower a portion of thevehicle. In another example, the front wheels of a vehicle and the backwheels of the vehicle can be controlled to act against each other havingthe effect of raising the overall level of the vehicle. In particular,more/less torque can be provided to the front/rear wheels of a vehiclefor effectively pushing the suspension arms of the wheels of the frontand rear of the vehicle toward each other, thereby causing them both torise up.

In accordance with an embodiment of the invention, inputs to a controlsystem such as the traction and/or suspension control system can be madevia steering wheel sensors and vehicle attitude and yaw sensors.

In one example, if a vehicle is in a skid management state (in which askid in one or more of the wheels has been detected) and the driverelects to move the steering wheel abruptly in one direction, thismovement magnitude and rate of change can be communicated to the controldevices described above so that appropriate compensating action can betaken by the control devices in their traction control strategy.Accordingly, a feed forward function of drivers intentions can be addedto the wheel drive software strategy to better handle a skid and respondto the drivers commands Because of the increased responsiveness of anelectric motor of the kind described above, the response of theindividual wheels can be much faster than that of existing systems.Moreover, since each wheel responds according to its local conditions,the possibility of more stable control exists when compared to existingsystems.

The traction control and suspension control systems described above maybe implemented using software executing on a microprocessor. Thesoftware can be provided on a carrier medium.

Magnet Mounting

A particularly effective manner of magnet mounting is shown in FIG. 24.As already described, magnets are mounted around the insidecircumferential wall 221 of the rotor 252. The magnets transfer all ofthe force from the magnetic field of the coils to the rotor and so, wehave appreciated, must be securely mounted to the circumferential rotorwall 221.

A portion of the circumferential wall, including magnets 242, is shownas enlargement A in FIG. 24. At each magnet position, there are actuallythree separate magnets 401, 402 and 403, which are inserted intodovetail slots 405 of the rotor back iron 407. This arrangement has anumber of advantages. First, by using separate magnet portions to createa single magnet at each magnet position, eddy currents in the magnetscan be reduced. Whilst three separate magnet portions are preferred,other numbers would be perfectly possible. Second, the dovetail mountingarrangement ensures exact location of the magnets on the circumferentialwall 221 and also ensures that they are securely mounted. Other slottype arrangements would be possible, but the dovetail mountingarrangement is preferred.

Cooling Arrangement

A particularly effective configuration for cooling the coils is shown inFIG. 25. This view of the stator is a cross section in the plane of theaxis of the stator. Referring briefly to FIG. 3, the coil teeth 235 ofFIG. 3 are shown in cross section in FIG. 25 with a single coil tooth514 shown, which can receive windings, which fit in the winding spaces510 and 512. As described in relation to FIGS. 16 and 17, the teeth aremounted on a back iron 150. The back iron is mounted directly on acooling plate 516, which, together with other walls, defines coolingchannels 502 and 504, through which cooling fluid can circulate eitherby convection or by pumping. In addition, a cooling channel 508 on oneside of the coils in space 512 cools the coils on one side of the teethand a cooling channel 506 on the opposite side cools the coils in space510. Accordingly, the coils are cooled by this multi faceted coolingplate, which encloses the windings on three sides. In addition, thefaces of the cooling channels provide for the attachment of electronicpower devices such as the control devices 80, dump resistor and so on. Asingle three sided cooling arrangement thus provides the cooling for thecoils, as well as the associated components from which heat isgenerated.

The whole assembly of the coils is potted onto the cooling plate using athermally conductive material but electrically insulating material, suchas epoxy filled with aluminium oxide. This improves the thermalconductivity from the coils to the back iron and to the cooling plate.

The invention claimed is:
 1. An electric motor for mounting in a wheelfor a road vehicle, the electric motor comprising: a stator comprising aradial wall and a plurality of coils; a rotor comprising: a radial walland a plurality of magnets surrounding the coils of the stator; and acircumferential wall on which the plurality of magnets are mounted onthe inside, the outside of the circumferential wall of the rotor havingno load bearing portions; the radial wall of the stator comprising acentral region for mounting to a vehicle on one side and for connectionto a bearing block; and the radial wall of the rotor comprising acentral region for mounting a wheel on the outside and to the bearingblock on the inside for connection to the vehicle by the bearing block.2. The electric motor of claim 1, wherein the radial wall of the rotorhas no load bearing portions.
 3. The electric motor of claim 1 mountedwithin a wheel, the wheel being mounted to the central region of theradial wall of the rotor.
 4. The electric motor and wheel of claim 3,wherein there is a gap between the wheel and an outer portion of theradial wall of the rotor.
 5. The electric motor and wheel of claim 3,wherein there is a gap between the circumferential wall of the rotor andthe wheel.
 6. A vehicle comprising a wheel and an electric motor formounting the wheel to the vehicle, the electric motor comprising: astator comprising a radial wall and a plurality of coils; a rotorcomprising: the radial wall of the stator comprising a central regionfor mounting to the vehicle on one side and for connection to a bearingblock; and the radial wall of the rotor comprising a central region formounting the wheel on the outside and to the bearing block on the insidefor connection to the vehicle by the bearing block.